Aromatics transalkylation to ethylbenzene and xylenes

ABSTRACT

The use of transalkylation catalysts to react heavy aromatic compounds of carbon number nine (and heavier carbon numbers) with benzene to form carbon number eight aromatics is disclosed. The catalyst system preserves ethyl-group species on the heavier aromatics that are otherwise de-ethylated over most gas-phase transalkylation catalysts to form undesired ethane gas with benzene or toluene. The catalyst system also promotes methyl-group species transalkylation at selected conditions. Thus, by using the transalkylation system, a greater yield of para-xylene or other carbon number eight aromatics may be achieved overall within an integrated aromatics complex.

FIELD OF THE INVENTION

This invention relates to an improved process for the conversion ofaromatic hydrocarbons. More specifically, the present invention concernsa liquid-phase process for transalkylation of benzene with C₉ ⁺alkylaromatics to directly obtain xylenes and ethylbenzene that wouldotherwise be lost via de-alkylation to benzene or toluene in aconventional gas-phase transalkylation process. The invention alsoincreases yields of xylenes when combined with another transalkylationand/or isomerization process.

BACKGROUND OF THE INVENTION

The xylene isomers are produced in large volumes from petroleum asfeedstocks for a variety of important industrial chemicals. The mostimportant of the xylene isomers is para-xylene, the principal feedstockfor polyester, which continues to enjoy a high growth rate from largebase demand. Ortho-xylene is used to produce phthalic anhydride, whichsupplies high-volume but relatively mature markets. Meta-xylene is usedin lesser but growing volumes for such products as plasticizers, azodyes and wood preservers. Ethylbenzene generally is present in xylenemixtures and is occasionally recovered for styrene production, but isusually considered a less-desirable component of C₈ aromatics.

Among the aromatic hydrocarbons, the overall importance of xylenesrivals that of benzene as a feedstock for industrial chemicals. Xylenesand benzene are produced from petroleum by reforming naphtha but not insufficient volume to meet demand, thus conversion of other hydrocarbonsis necessary to increase the yield of xylenes and benzene. Often tolueneis de-alkylated to produce benzene or selectively disproportionated toyield benzene and C₈ aromatics from which the individual xylene isomersare recovered.

A current objective of many aromatics complexes is to increase the yieldof xylenes and to de-emphasize benzene production. Demand is growingfaster for xylene derivatives than for benzene derivatives. Refinerymodifications are being effected to reduce the benzene content ofgasoline in industrialized countries, which will increase the supply ofbenzene available to meet demand. A higher yield of xylenes at theexpense of benzene thus is a favorable objective, and processes totransalkylate C₉ aromatics and toluene have been commercialized toobtain high xylene yields.

U.S. Pat. No. 4,459,426 (Inwood et al.) discloses a liquid-phasetransalkylation process, which is used in conjunction with an olefinalkylation process, that converts a poly-alkylaromatic mixture intoadditional mono-alkylaromatic compounds, such as ethylbenzene. Thisdisclosure teaches that only trace amounts of xylenes, which are highlyundesirable for such a process, are produced in amounts less than 0.2wt-percent.

U.S. Pat. No. 5,004,855 (Tada et al.) discloses a process forethylbenzene destruction within a C₈ alkylaromatic mixture. U.S. Pat.No. 6,342,649 B1 (Winters et al.) also discloses a method of removingethylbenzene from a C₈ alkylaromatic mixture. Both of these disclosuresteach conversion of the ethylbenzene component to benzene byirreversible de-ethylation.

Other types of transalkylation processes have been disclosed. U.S. Pat.No. 5,847,256 (Ichioka et al.) discloses a process for producing xylenefrom a feedstock containing C9 alkylaromatics with ethyl-groups over acatalyst containing a zeolite component that is preferably mordenite andwith a metal component that is preferably rhenium. U.S. Pat. No.5,942,651 (Beech, Jr. et al.) discloses a flowscheme for a gas-phasetransalkylation process in the presence of two zeolite containingcatalysts to produce xylenes and benzene. The first catalyst contains ahydrogenation metal component and a zeolite component from the groupincluding MCM-22, PSH-3, SSZ-25, ZSM-12, and zeolite beta. The secondcatalyst contains ZSM-5, and is used to reduce the level of saturateco-boilers necessary for a high-purity benzene product. U.S. Pat. No.5,952,536 (Nacamuli et al.) discloses a gas-phase transalkylationprocess using a catalyst comprising a zeolite from the group includingSSZ-26, Al-SSZ-33, CIT-1, SSZ-35, and SSZ-44. The catalyst alsocomprises a mild hydrogenation metal such as nickel or palladium, and isused to convert aromatics with at least one alkyl group includingbenzene.

Economical processes in the field of integrated aromatics complexes arecontinually sought having exceptionally high selectivity for xylenesfrom other aromatic intermediates.

SUMMARY OF THE INVENTION

Accordingly, one embodiment of the present invention is directed to aprocess using transalkylation catalysts to covert heavy aromaticcompounds of carbon number nine (and heavier carbon numbers), otherwisecalled C₉ ⁺ alkylaromatics, with benzene to form carbon number eightaromatics. The catalyst system preserves ethyl-group species on theheavier aromatics that are otherwise de-ethylated over most gas-phasetransalkylation catalysts to form undesired ethane gas with benzene ortoluene. The catalyst system also promotes methyl-group speciestransalkylation at conditions of at least partial liquid phase todirectly and economically produce valuable xylenes. Such a liquid phaseprocess offers obvious advantages over a gas phase process in capitalrequirements, such as the elimination of a phase separator vessel and arecycle gas compressor.

In another embodiment of the present invention, a process fortransalkylation of benzene and C₉ ⁺ alkylaromatics is integrated with aseparate transalkylation process. The integrated process increasesselectivity to xylenes by addressing the preservation of ethylbenzene ina first transalkylation unit that would otherwise be lost in a secondtransalkylation unit, which results in a higher overall yield ofvaluable xylenes from both units. Preferably, the first transalkylationunit is substantially liquid phase, while the second separatetransalkylation process is substantially gas phase.

In yet another embodiment of the present invention, a greater yield ofpara-xylene or other carbon number eight aromatics may be achievedoverall within an integrated aromatics complex using a para-xyleneproduction unit.

Additional objects, embodiments and details of this invention can beobtained from the following detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE schematically illustrates the major equipment used inperforming the process of this invention. In the process C₉ ⁺alkylaromatics (called A₉ ⁺) carried by a line 10 is admixed withbenzene from a line 12 to form a combined line 14 and enters atransalkylation reactor 16. After contact with a zeolitic catalyst, aline 18 carries the effluent from the transalkylation reactor 16 to acombination point with a second transalkylation product stream in a line20 to form a combined stream in a line 22 that enters a separationcolumn 24. Separation column 24 separates the combined stream into anoverhead of benzene taken by a line 26; a bottoms stream of C₈ ⁺alkylaromatics including ethylbenzene and xylene taken by a line 32; anda sidecut stream of toluene removed by a line 30. The overhead stream inline 26 is recycled back to transalkylation reactor 16 by line 12 afterbenzene is either removed or added via a line 28. The bottoms stream inline 32 is flowed to a second separation column 38 from which anoverhead stream of ethylbenzene and xylene is taken by a line 40 and abottom stream of C₉ ⁺ alkylaromatics is withdrawn by a line 42. Theethylbenzene and xylene stream is sent via line 40 to a para-xyleneproduction unit 66 to produce para-xylene by a line 68. The sidecutstream in line 30 is ultimately recycled to a second transalkylationreactor 58 by a line 46 after toluene is either removed or added via aline 44. Toluene in line 46 is admixed with line 42 to form a combinedline 48 that enters a third separation column 50. Separation column 50separates the combined stream into bottoms stream of C₁₁ ⁺alkylaromatics (called heavies) withdrawn by a line 52, and an overheadstream of C₁₀, C₉ alkylaromatics, and lighter compounds (including C₇alkylaromatics) carried by a line 54 to second transalkylation reactor58. Hydrogen is added to second transalkylation reactor 58 via a line56. After contact with a transalkylation catalyst, a line 60 carries theeffluent to a stabilizer column 64 from which an overhead stream oflight end hydrocarbons (called light-ends gas, which generally comprisesat least ethane) is taken by a line 62 and a bottom stream of secondtransalkylation product is withdrawn by line 20.

DETAILED DESCRIPTION OF THE INVENTION

The feedstream to the present process generally comprises alkylaromatichydrocarbons of the general formula C₆H_((6-n))Rn, where n is an integerfrom 0 to 5 and each R may be CH₃, C₂H₅, C₃H₇, or C₄H₉, in anycombination. Suitable alkylaromatic hydrocarbons include, for examplebut without so limiting the invention, benzene, toluene, ortho-xylene,meta-xylene, para-xylene, ethylbenzene, ethyltoluenes, propylbenzenes,tetramethylbenzenes, ethyl-dimethylbenzenes, diethylbenzenes,methylpropylbenzenes, ethylpropylbenzenes, triethylbenzenes,di-isopropylbenzenes, and mixtures thereof.

The feed stream preferably comprises benzene and C₉ ⁺ aromatics andsuitably is derived from one or a variety of sources. The molar ratio ofbenzene to C₉ ⁺ aromatics is preferably from about 0.5 to about 10 andeven more preferably from about 1 to about 6. Feedstock may be producedsynthetically, for example, from naphtha by catalytic reforming or bypyrolysis followed by hydrotreating to yield an aromatics-rich product.The feedstock may be derived from such product with suitable purity byextraction of aromatic hydrocarbons from a mixture of aromatic andnonaromatic hydrocarbons and fractionation of the extract. For instance,aromatics may be recovered from a reformate stream. The reformate streammay be produced by any of the processes known in the art. The aromaticsthen may be recovered from the reformate stream with the use of aselective solvent, such as one of the sulfolane type, in a liquid—liquidextraction zone. The recovered aromatics may then be separated intostreams having the desired carbon number range by fractionation. Whenthe severity of reforming or pyrolysis is sufficiently high, extractionmay be unnecessary and fractionation may be sufficient to prepare thefeedstock. Benzene may also be recovered from the product oftransalkylation.

A preferred component of the feedstock is a heavy-aromatics streamcomprising C₉ ⁺ aromatics. C₁₀ ⁺ aromatics also may be present,typically in an amount of 50 wt-% or less of the feed. Theheavy-aromatics stream generally comprises at least about 90 wt-%aromatics, and may be derived from the same or different known refineryand petrochemical processes as the benzene and toluene feedstock and/ormay be recycled from the separation of the product from transalkylation.

The feedstock is preferably transalkylated in the liquid-phase and inthe substantial absence of hydrogen. Substantial absence of hydrogenmeans without the addition of hydrogen beyond what may already bepresent and dissolved in a typical liquid aromatics feedstock. In thecase of partial liquid phase, hydrogen may be added in an amount lessthan 1 mole per mole of alkylaromatics. If the feedstock istransalkylated in the gas-phase, then hydrogen is added with thefeedstock and recycled hydrocarbons in an amount of from about 0.1 molesper mole of alkylaromatics up to 10 moles per mole of alkylaromatic.This ratio of hydrogen to alkylaromatic is also referred to as hydrogento hydrocarbon ratio. The transalkylation reaction preferably yields aproduct having increased xylene content of at least greater than 1 wt-%and also comprises ethylbenzene. The yield of 1 wt-% xylene iscalculated on a net effluent basis. More preferably, the product is atleast 3 wt-% calculated on the net effluent basis. When hydrogen isadded to a transalkylation unit, the unit preferably comprises a recyclegas compressor to assist in recycling of hydrogen recovered from thereactor effluent in a separator vessel.

Generally, the use of two transalkylation zones will provide betterresults then the use of one transalkylation zone. When two zones areused, better results may be obtained when one zone is liquid-phase andone zone is gas-phase. Each transalkylation zone will continue to bedescribed in generic terms below. Note that details of heat exchange andadditional flow details within the zones have not been shown in theschematic FIGURE because they are well known to the art.

The feed to a transalkylation reaction zone usually first is heated byindirect heat exchange against the effluent of the reaction zone andthen is heated to reaction temperature by exchange with a warmer stream,steam or a furnace. The feed then is passed through a reaction zone,which may comprise one or more individual reactors. The use of a singlereaction vessel having a fixed cylindrical bed of catalyst is preferred,but other reaction configurations utilizing moving beds of catalyst orradial-flow reactors may be employed if desired. Passage of the combinedfeed through the reaction zone effects the production of an effluentstream comprising unconverted feed and product hydrocarbons. Thiseffluent is normally cooled by indirect heat exchange against the streamentering the reaction zone and then further cooled through the use ofair or cooling water. The effluent may be passed into a stabilizer orstripping column in which substantially all C₅ and lighter hydrocarbonspresent in the effluent are concentrated into an overhead stream andremoved from the process. An aromatics-rich stream is recovered as a netcolumn bottoms stream which is referred to herein as the transalkylationeffluent or transalkylation product.

To effect a transalkylation reaction, the present invention incorporatesa transalkylation catalyst in at least one zone. Conditions employed inthe transalkylation zone normally include a temperature of from about100° to about 540° C. The transalkylation zone is operated at moderatelyelevated pressures broadly ranging from about 100 kPa to about 6 MPaabsolute. The transalkylation reaction can be effected over a wide rangeof space velocities. The weight hourly space velocity (WHSV) of thepresent invention generally is in the range of from about 0.1 to about20 hr⁻¹. Preferably, these transalkylation conditions comprise atemperature from about 200° to about 300° C., a pressure from about 10to about 50 kg/cm², and a weight hourly space velocity from about 0.5 toabout 15 hr^(−1.)

The transalkylation effluent is separated into a light recycle stream, amixed C₈ aromatics product and a heavy-aromatics stream. The mixed C₈aromatics product can be sent for recovery of para-xylene and othervaluable isomers. The light recycle stream may be diverted to other usessuch as to benzene and toluene recovery, but alternatively is recycledpartially to the transalkylation zone. The heavy recycle stream containssubstantially all of the C₉ and heavier aromatics and may be partiallyor totally recycled to the transalkylation reaction zone.

One skilled in the art is familiar with several types of transalkylationcatalysts that may be suitably used in the present invention. Forexample, in U.S. Pat. No. 3,849,340, which is herein incorporated byreference, a catalytic composite is described comprising a mordenitecomponent having a SiO₂/Al₂O₃ mole ratio of at least 40:1 prepared byacid extracting Al₂O₃ from mordenite prepared with an initial SiO₂/Al₂O₃mole ratio of about 12:1 to about 30:1 and a metal component selectedfrom copper, silver and zirconium. U.S. Pat. No. 4,083,866 is alsoincorporated by reference, and describes a process for transalkylationof alkylaromatic hydrocarbons that uses a zeolitic catalyst.Friedel-Crafts metal halides such as aluminum chloride have beenemployed with good results and are suitable for use in the presentprocess. Hydrogen halides, boron halides, Group I-A metal halides, irongroup metal halides, etc., have been found suitable. Refractoryinorganic oxides, combined with the above-mentioned and other knowncatalytic materials, have been found useful in transalkylationoperations. For instance, silica-alumina is described in U.S. Pat. No.5,763,720, which is incorporated herein by reference.

Crystalline aluminosilicates have also been employed in the art astransalkylation catalysts. Examples of zeolites that are particularlysuited for this purpose include, but are not limited to, zeolite beta,zeolite MTW, zeolite Y (both cubic and hexagonal forms), zeolite X,mordenite, zeolite L, zeolite ferrierite, MFI, and erionite. Zeolitebeta is described in U.S. Pat. No. 3,308,069 according to its structure,composition, and preferred methods of synthesis. Y zeolites are broadlydefined in U.S. Pat. No. 3,130,007, which also includes synthesis andstructural details. Mordenite is a naturally occurring siliceous zeolitewhich can have molecular channels defined by either 8 or 12 memberrings. Donald W. Breck describes the structure and properties ofmordenite in Zeolite Molecular Sieves (John Wiley and Sons, 1974, pp.122–124 and 162–163). Zeolite L is defined in U.S. Pat. No. 3,216,789,which also provides information on its unique structure as well as itssynthesis details. Other examples of zeolites that can be used are thosehaving known structure types, as classified according to theirthree-letter designation by the Structure Commission of theInternational Zeolite Association (“Atlas of Zeolite Structure Types”,by Meier, W. M.; Olsen, D. H; and Baerlocher, Ch., 1996) of MFI, FER,ERI, and FAU. Zeolite X is a specific example of the latter structuretype that may be used in the present invention. The zeolite structuretype MTW is also suitable.

A refractory binder or matrix is optionally utilized to facilitatefabrication of the catalyst, provide strength and reduce fabricationcosts. The binder should be uniform in composition and relativelyrefractory to the conditions used in the process. Suitable bindersinclude inorganic oxides such as one or more of alumina, magnesia,zirconia, chromia, titania, boria, thoria, phosphate, zinc oxide andsilica.

The zeolite may be present in a range from 5 to 99 wt-% of the catalystand the refractory inorganic oxide may be present in a range of fromabout 5 to 95 wt-%. Preferred transalkylation catalysts are either atype Y zeolite having an alumina or silica binder, or a beta zeolitehaving an alumina or silica binder. Alumina is an especially preferredinorganic oxide binder for both zeolite compositions.

The catalyst also contains an optional metal component. One preferredmetal component is a Group VIII (IUPAC8–10) metal, preferably aplatinum-group metal, i.e., platinum, palladium, rhodium, ruthenium,osmium and iridium, Alternatively a preferred metal component isrhenium. Of the preferred platinum-group metals, platinum metal itselfis especially preferred. This optional metal component may exist withinthe final catalytic composite as a compound such as an oxide, sulfide,halide, or oxyhalide, in chemical combination with one or more of theother ingredients of the composite, or, preferably, as an elementalmetal. This component may be present in the final catalyst composite inany amount which is catalytically effective, generally comprising about0.01 to about 2 wt-% of the final catalyst calculated on an elementalbasis. The component may be incorporated into the catalyst in anysuitable manner such as coprecipitation or cogelation with the carriermaterial, ion exchange or impregnation. Impregnation using water-solublecompounds of the metal is preferred, for example with chloroplatinicacid or perrhenic acid. Rhenium may also be used in conjunction with aplatinum-group metal.

The catalyst may optionally contain a modifier component. Preferredmetal modifier components of the catalyst include, for example, tin,germanium, lead, indium, and mixtures thereof. Catalytically effectiveamounts of such metal modifiers may be incorporated into the catalyst byany suitable manner. A preferred amount is a range of about 0.01 toabout 2.0 wt-% on an elemental basis.

Generally, water may have a deleterious effect on the catalyst andprolonged contact with the catalyst will cause a loss of activity asdescribed in U.S. Pat. No. 5,177,285 and U.S. Pat. No. 5,030,786. Thus,a typically low water concentration of less than about 200 wt-ppmresults in reasonable operation.

An aromatics complex flow scheme has been disclosed by Meyers in theHandbook of Petroleum Refining Processes, 2d. Edition in 1997 byMcGraw-Hill, which is incorporated by reference, based upon aconventional gas-phase transalkylation unit located within an integratedaromatics complex flow scheme designed for para-xylene production.Gas-phase herein means units that require addition of hydrogen, andgenerally contain hydrogen gas phase recycle loop systems around areactor system.

An integrated aromatics complex will generally incorporate thetransalkylation unit of the present invention along with a reformingunit, an alkyl-aromatic isomerization unit, a para-xylene separationunit, and an optional second transalkylation unit. The reforming unitwill be used to generate the aromatic species that may be furtherseparated in other units. Benzene is transalkylated in combination withA₉+ aromatics to form xylenes and ethylbenzene in the transalkylationunit. Toluene may be further transalkylated in the optional secondtransalkylation unit to form additional xylenes in a transalkylationunit which are then processed in a loop comprising the isomerization andpara-xylene separation units. The para-xylene separation unit may beeither a crystallization or adsorptive based separation process wellknown to the art, which selectively removes the para-xylene in highpurity while rejecting a non-equilibrium mixture of other xylenes andethylbenzene. The non-equilibrium mixture, depleted in para-xylene, iscontacted with an alkylaromatic isomerization catalyst in anotherprocess well-known in the art. The isomerization process re-equilibratesthe mixture back to an equilibrium amount of para-xylene and convertsethylbenzene to xylenes which can be recycled back to the para-xyleneseparation unit for further recovery. Often the combination of apara-xylene separation unit and an alkylaromatic isomerization unit iscalled a ‘loop’. This loop is defined herein as a ‘para-xyleneproduction’ unit, wherein the loop produces para-xylene, which isrecovered as a product from the process.

EXAMPLE I

An increased selectivity to A₈s at the expense of light ends has beendemonstrated in pilot plant tests and is shown in the following materialbalance comparison. The prior art, gas-phase transalkylation process, iscompared against the present invention, which combines a liquid-phasetransalkylation process with a gas-phase process. This comparison showsthe benefits of the present invention as increased xylenes andethylbenzene, and concomitantly decreased benzene and light-end gas(especially ethane). By reducing the production of ethane byde-ethylation in gas-phase reactions within an aromatics complex, theinvention provides improved total retention of aromatics relative toprior art transalkylation units used in complexes that produce xylenes.

With reference to the FIGURE, showing the flow scheme of the presentinvention, a simulated material balance is shown below. The liquid-phasetransalkylation process unit is combined with the gas-phasetransalkylation process unit, and results in the following changes overa prior art single gas-phase transalkylation unit. Hydrogen feed to theflow scheme decreases. Feed of toluene and A9+ remains constant.Production of A₈s increases, while benzene production decreases. Heaviesproduction remains constant. Most importantly, light-end gas productiondecreases.

These changes are summarized in the following table:

Single Gas-phase Two Transalkylation Units Transalkylation Unit (asshown in FIGURE) Feed (kMTA) A9+ 151.7 151.7 Hydrogen (H2) 2.8 2.0Toluene 151.7 151.7 Total 306.1 305.4 Product (kMTA) A₈ 208.1 245.5Benzene 53.1 25.8 Light-end Gas 33.4 22.6 Heavies 11.5 11.5 Total 306.1305.4

Thus, the flow scheme of the present invention provides a benefit byproducing more of the desirable A₈ material, which is the valuablexylenes and ethylbenzene.

EXAMPLE II

The unexpected transalkylation of methyl groups along with ethyl groupsat the expense of light ends has been demonstrated in pilot plant testswith cylindrical down-flow liquid-phase reactors operated at a pressureof about 35 kg/cm² and is shown in the following results. A stream of A₉⁺ comprising about 75-wt % A₉ and about 25 wt-% A₁₀ with an endpointaround 200° C., contacted one of two catalysts comprising a zeoliticaluminosilicate of type Y or type Beta. A pure benzene stream wascombined and fed to each catalyst in a different molar ratio with a A9+stream. These results are summarized in the following table:

Y Beta Catalyst Temperature (° C.) 250 250 WHSV (hr⁻¹)  9.5  9.5Benzene/A₉ ⁺ (mol/mol)  4.3  1.1 Product Net Wt-% Ethylbenzene  4.4  5.7Toluene  5.4  7.2 Xylenes  3.3  4.2 Non-Aromatics  0.2  0.3 Heavies  1.4 2.5 Benzene+A₉+A₁₀ &  4.9 (7.8)  20.3 (18) (Tri-methylbenzene)conversion

Thus, the present invention provides a benefit by directly producingmore of the desirable xylenes material and indirectly producingethylbenzene, which can be isomerized to additional xylenes in anotherunit. The amount of tri-methylbenzene (TMB) converted under theseconditions was also surprising.

1. A process for conversion of aromatic hydrocarbons into para-xylenecomprising: a) providing a stream including benzene and C₉ ⁺alkylaromatics; b) passing the stream of step (a) to a liquid-phasetransalkylation unit, wherein said streams are contacted with a firsttransalkylation catalyst under first transalkylation conditions withoutadded hydrogen to produce a first transalkylation product streamcomprising ethylbenzene and at least 1 wt-% xylene calculated on a neteffluent basis; c) combining at least a portion of thefirst-transalkylation-product stream with asecond-transalkylation-product stream; d) separating the combinedstreams of step (c) in a fractionation zone comprising at least onecolumn to produce a fractionated-benzene stream and a xylene-plusstream; e) separating the xylene-plus stream in a xylene column toproduce a xylene enriched stream and a C₉ ⁺ alkylaromatic-enrichedstream; f) passing the xylene enriched stream to a para-xyleneproduction unit to produce para-xylene, which is recovered as a productfrom the process; g) providing a toluene stream; h) combining thetoluene stream with at least part of the C₉ ⁺ alkylaromatic-enrichedstream of step (e); i) separating the combined streams of step (h) in aheavy aromatics column to produce a stream rich in C₇, C9 and C₁₀alkylaromatics and a stream rich in C₁₁ ⁺ alkylaromatics; j) passing thestream rich in C₇, C₉ and C₁₀ alkylaromatics to a gas-phasetransalkylation unit, wherein said stream is contacted with a secondtransalkylation catalyst under second transalkylation conditions toproduce ar unstabilized transalkylation stream; and k) separating theunstabilized transalkylation stream in a transalkylation stabilizercolumn to produce a gas stream and the second transalkylation productstream of step (c).
 2. The process of claim 1 wherein thefirst-transalkylation catalyst comprises an inorganic oxide binder and azeolitic aluminosilicate selected from the group consisting of MTW, MFI,type Y, beta, and mordenite.
 3. The process of claim 1 wherein thesecond-transalkylation catalyst comprises a zeolitic aluminosilicateselected from the group consisting of MTW, MFI, type Y, beta, andmordenite, an inorganic oxide binder, and an optional metal component.4. The process of claim 1 wherein the first-transalkylation orsecond-transalkylation conditions comprise a temperature from about 100°to about 540° C., a pressure from about 1 to about 60 kg/cm², and aweight hourly space velocity from about 0.1 to about 20 hr⁻¹.
 5. Theprocess of claim 4 wherein the second-transalkylation conditions furthercomprise a hydrogen to alkylaromatic ratio of about 0.1 moles per moleof alkylaromatics up to about 10 moles per mole of alkylaromatic.
 6. Theprocess of claim 4 wherein the conditions further comprise a waterconcentration of less than about 200 wt-ppm.
 7. The process of claim 1wherein the fractionation zone of step (d) produces afractionated-toluene stream.
 8. The process of claim 7 wherein thefractionated-toluene stream is substituted for at least part of thetoluene stream of step (g).
 9. The process of claim 7 wherein thecombined streams of step (h) are combined with the fractionated-toluenestream.
 10. The process of claim 1 further comprising the step ofrecycling at least part of the fractionated-benzene stream of step (d)back to the benzene stream of step (a).